Implants for placement in the intervertebral space between adjacent vertebral bodies in the spine come in a wide range of shapes and sizes. These implants are usually made entirely of one material, although the type of material can vary significantly between specific implants. Such implants for use in human spinal surgery include implants made entirely of metals, such as titanium or stainless steel, or synthetic radiolucent materials such as carbon-carbon composites or poly-ether-ether-ketone (PEEK). Implants may have a structure designed to promote fusion across adjacent vertebral bodies by allowing bone to grow through and around the implant. The operative placement of intervertebral implants is optimized by radiographic opacity. However, a relatively radiolucent implant material optimizes postoperative evaluation of bone growth and fusion across an intervertebral space. While these implants may contain marking beads or radio opaque markers they do not structurally benefit from radio opaque materials. In some configurations, metals, some of which are opaque on radiographs, provide greater strength and resistance to impaction during implantation. Metallic implants may offer reduced wall thickness of structural components and offer increased volume for bone graft and other agents within an implant.
As it is desirable to take advantage of benefits of radiolucent and radio-opaque materials in an implant, there exists a need for an improved implant made of different structural materials with different properties of radiographic appearance. For some implants, it is desirable to provide optimization of mechanical properties, while permitting generous bone filling and bone through-growth. These characteristics may be applied in some embodiments in combination with an ability to radiographically determine bone-implant interaction and bone growth into and around the implant.